Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique in which a user transmits on many orthogonal frequencies (or subcarriers). The orthogonal subcarriers are individually modulated and separated in frequency such that they do not interfere with one another. This provides high spectral efficiency and resistance to multipath effects. An orthogonal frequency division multiple access (OFDMA) system allows some subcarriers to be assigned to different users, rather than to a single user.
A Discrete Fourier Transform-spread OFDM system is particularly attractive for use in the uplink channel from a subscriber station to the base station. This is because a DFT-spread OFDM system has low peak-to-average power ratio (PAPR) characteristics. A key design parameter for OFDM and OFDMA power amplifiers is the requirement to minimize the generation of adjacent channel power (ACP) noise—undesired spectral components in adjacent transmission channels. Power amplifiers are more linear at lower input signal levels. Large input signals tend to drive a power amplifier into saturation. Thus, smaller signals experience less distortion and have lower ACP noise.
However, a sudden large signal peak still results in distortion and ACP noise. This is especially problematic in systems that have large peak-to-average power ratios (PAPRs), such as OFDM and OFDMA systems. To avoid this, power amplifiers often operate in back-off (BO) mode (i.e., reduced input signal) in order to accommodate large signal peaks. DFT-spread OFDM minimizes the PAPR in the uplink, which allows the subscriber station amplifier to operate with a smaller amount of back-off and provides the subscriber stations with higher transmit power and higher data rate.
Therefore, there is a need for improved multi-carrier (e.g., OFDM, OFDMA) transmission systems that minimize amplifier peak-to-average power ratio (PAPR) without suffering performance degradation. In particular, there is a need to maximize the performance of DFT-spread OFDM transmission techniques.